Recently, Xerox Corporation, Stamford Connecticut, commercially introduced a facsimile device, sold under the trademark Xerox.RTM. Telecopier.RTM. 200 transceiver, which records on plain paper. The transceiver employs a low-energy helium-neon laser and uses the xerographic principle to receive and print messages on ordinary, unsensitized paper. The transceiver unit operates at the rate of 2 minutes per letter-sized document and includes alternate speed options which makes the unit compatible with other transceivers which operate at 4 and 6 minutes. Basically, when the transceiver is in the transmit mode, the laser provides a small stable beam of light to raster scan the original document. The reflected light is detected by a photosensor which translates the white and black of the document to electrical logic levels which may be transmitted by a phone line to a remote transceiver set to the receive mode. The receiver transceiver directs the laser beam onto a xerographic drum and by electrically modulating the laser with "1" and "0" logic levels in synchronism with the transmitter produces a copy of the original.
It would be desired that in addition to digitally modulating the laser beam to the full power level (maximum lasing beam intenstiy) or minimum power level (non-lase condition) that the laser beam would be controlled in a manner whereby the beam intensity is variable between the maximum and minimum power levels. This would allow the transceiver to operate at various switch selectable speeds without the use of additional optical and mechanical elements. In particular, at very slow speeds (high resolution) less laser beam intensity is required to write on the xerographic drum whereas for very high speeds (less resolution) greater laser beam intensity is required. The problem has been solved by introducing neutral density filters mechanically into the beam path as one selected the various operating speeds. However, the selection of the proper filter for a given transceiver unit and speed is very time consuming. Therefore, it has become apparent that an additional input signal that would electronically control the maximum intensity of the laser beam in accordance with the transceiver operating speed would be required. Further, if the additional input signal is analog, an additional benefit would be provided in that the analog input will linearly modulate the beam intensity between the minimum and maximum power levels whereby grey scale laser printing is provided. That is, the incoming data stream into the receive transceiver may contain information to turn on the laser beam and also indicate what the intensity of the laser beam should be similar to the manner in which television pictures are produced.
The prior art laser modulating devices, in general, modulate laser beams indirectly through optical manipulation, i.e., manipulating the light beam after it has left the laser. This technique is limited by the obvious difficulties of requiring additional optical hardware as well as the difficulty of coupling a high frequency modulating signal to the optical modulating means.
Direct modulation of a laser tube by varying the laser output power is described in U.S. Pat. No. 3,806,762. This patent describes a technique which utilizes a ballast resistor to reduce power dissipation by appropriate selection of impedance levels when one of two parallel impedance paths are coupled to the laser tube. The aforementioned patent briefly describes a technique for modulating the tube to provide different power levels between the on and off power levels by switching on a different current source, or a combination of such current sources. This technique therefore requires a plurality of current sources, coupled in a cascade arrangement, to provide an effective continuous analog control of the laser beam intensity.